In recent years, various techniques for crystallizing or improving the crystallinity of an amorphous or polycrystalline semiconductor film have been investigated. This technology is used in the manufacture of a variety of devices, such as image sensors and displays, for example, active-matrix liquid-crystal display (AMLCD) devices. In the latter, a regular array of thin-film transistors (TFTs) are fabricated on an appropriate transparent substrate, and each transistor serves as a pixel controller.
Semiconductor films are processed using excimer laser annealing (ELA), also known as line beam ELA, in which a region of the film is irradiated by an excimer laser to partially melt the film and then is crystallized. FIG. 1A illustrate low-temperature poly-silicon (poly-si) (LTPS) microstructures that can be obtained by laser induced melting and solidification. The process typically uses a long, narrow beam shape that is continuously advanced over the substrate surface, so that the beam can potentially irradiate the entire semiconductor thin film in a single scan across the surface. ELA produces small-grained polycrystalline films; however, the method often suffers from microstructural non-uniformities which can be caused by pulse to pulse energy density fluctuations and/or non-uniform beam intensity profiles. FIG. 2 is an image of the random microstructure that results from ELA. The Si film is irradiated multiple times to create the random polycrystalline film with a uniform grain size.
Sequential lateral solidification (SLS) using an excimer laser is one method that has been used to form high quality polycrystalline films having large and uniform grains. SLS is a crystallization process that provides elongated grains of a crystallized material in predefined locations on a film. FIGS. 1B-1D illustrate microstructures that can be obtained by SLS. A large-grained polycrystalline film can exhibit enhanced switching characteristics because the reduced number of grain boundaries in the direction of electron flow provides higher electron mobility. SLS processing controls the location of grain boundaries. U.S. Pat. Nos. 6,322,625; 6,368,945; 6,555,449; and 6,573,531 issued to Dr. James Im, the entire disclosures of which are incorporated herein by reference, and which are assigned to the common assignee of the present application, describe such SLS systems and processes.
FIGS. 3A-3F illustrate the SLS process schematically. In an SLS process, an initially amorphous or polycrystalline film (for example, a continuous wave (CW)—processed Si film, an as-deposited film, or solid phase crystallized film) is irradiated by a very narrow laser beamlet. The beamlet is formed, for example, by passing a laser beam pulse through a slotted mask, which is projected onto the surface of the silicon film. The beamlet melts the amorphous silicon and, upon cooling, the amorphous silicon film recrystallizes to form one or more crystals. The crystals grow primarily inward from edges of the irradiated area toward the center. After an initial beamlet has crystallized a portion of the film, a second beamlet irradiates the film at a location less than the “lateral growth length” from the previous beamlet. In the newly irradiated film location, crystal grains grow laterally from the crystal seeds of the polycrystalline material formed in the previous step. As a result of this lateral growth, the crystals are of high quality along the direction of the advancing beamlet. The elongated crystal grains are generally perpendicular to the length of the narrow beamlet and are separated by grain boundaries that run approximately parallel to the long grain axes.
When polycrystalline material is used to fabricate electronic devices, the total resistance to carrier transport is affected by the combination of barriers that a carrier has to cross as it travels under the influence of a given potential. Due to the additional number of grain boundaries that are crossed when the carrier travels in a direction perpendicular to the long grain axes of the polycrystalline material or when a carrier travels across a larger number of small grains, the carrier will experience higher resistance as compared to the carrier traveling parallel to long grain axes. Therefore, the performance of devices fabricated on polycrystalline films formed using SLS, such as TFTs, will depend upon the crystalline quality and microstructure of the TFT channel relative to the long grain axes, which corresponds to the main growth direction.
To achieve acceptable system performance for devices that utilize a polycrystalline thin film there still remains a need to optimize manufacturing processes that provide a defined, crystallographic orientation of the crystal grains.